Kuhn closed the Introduction to The Structure of Scientific Revolutions with what was clearly intended as a rhetorical question: How could history of science fail to be a source of phenomena to which theories about knowledge may legitimately be asked to apply? (Kuhn 1970, 9) This paper argues that there is a more fruitful way of conceiving the relationship between a historical and philosophical study of science, which is dubbed the 'phylogenetic' approach. I sketch an example of this approach, and take a first pass at an abstract characterization of the method.